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4Results of theoretical research and experiments exploring novel energy statesIt was predicted over 40 years ago that if the coupling is extremely strong, a qualitatively new lowest energy state (the ground state) of light and an atom should be realized. A debate soon started as to whether this prediction would still apply when realistic conditions are considered. Our research collaborator Dr. S. Ashhab (QEERI) and others conducted theoretical study several years ago to dene conditions necessary to observe the novel ground state using a superconducting circuit [9].During our recent experiment, we prevented our sample from thermal excitation at the single microwave photon level. Accordingly, we used a dilution refrigerator (Fig. 3) to cool the sample. We used a superconducting articial atom with quantum properties similar to those of atoms made by microfabrication techniques and photons conned in a superconducting circuit. Specically, we de-signed a circuit composed of an LC resonance circuit with large zero-point uctuation current and a superconducting persistent-current qubit (Fig. 4). ese components share a large Josephson inductance*2, thereby enabling very strong magnetic coupling. We performed experiments in which we took spectroscopic measurements of the super-conducting electric circuit (transmission spectrum mea-surements at the single photon level as shown in Fig. 5) and analyzed the obtained spectra. As a result, we con-rmed a novel ground state as predicted [7]. e total energy of the articial atoms in the circuit is the sum among the energy of the light involved, the energy of the atoms and the interaction energy between the light and atoms. By taking advantage of superconducting articial atoms as the macroscopic quantum system*3, we succeeded to make light-atom interaction energy larger than the en-ergy of the light itself and the energy of the atoms them-selves (i.e., we achieved g > Δ, ω0 or the DSC regime).It had been previously observed that the interaction energy between light and atoms —especially that generated when atoms are inuenced by vacuum uctuations of the electromagnetic eld— produced only a tiny (less than 1 ppm) perturbation energy (a Lamb shi, for example) FiF4Superconducting artificial atom – LC resonator coupled systema. An equivalent circuit (“X” and “x” in the circuit represent Josephson junctions), an LC circuit (blue and black) and a superconducting artificial atom (red and black)b. Superconducting artificial atoms integrated into a part of the quantum LC circuit (red rectangle) The white parts are made of aluminum and the gray parts are silicon substrates.c. Superconducting artificial atom (enlarged view of the red rectangle in b)Four Josephson junctions are installed in parallel along the side shared by the artificial atom and the quantum LC circuit. The system’s SQUID structure enables it to behave like a variable inductor (black part in the equivalent circuit in a) when different external magnetic fields are ap-plied. As such, this system enables the realization of multiple different coupling strengths in the same sample. *2Josephson inductance refers to a superconducting state inductance of a Josephson junction, a device composed of two superconductors sandwiching a very thin (atomic level) barrier layer. When the junction —which weakens superconductivity— receives external electromagnetic signals, it generates nonlinear responses unique to superconducting states. The highly sensitive magnetic field sensor SQUID (superconducting quantum interference device) contains multiple Josephson junctions distributed along its superconducting loop. SQUID takes advantage of superconducting current flowing through the loop which is highly sensitive to the magnetic field penetrating the loop. In our recent experiment, we actively used the large inductance of Josephson junctions to achieve strong coupling. We fabricated Josephson junctions in the part of the LC resonance circuit which is shared with a superconducting artificial atom (the variable inductor indicated in black in the equivalent circuit in Fig. 4a). This arrangement strengthened the coupling between the atom and the circuit to reach the DSC regime.*3In a superconducting state, a tremendous number of electron pairs occupy the same quantum state. Under this condition, a phase-coherent quantum state emerges at a scale much larger than the atomic scale (i.e., the macroscopic scale). Accordingly, the use of superconductors can enable the current state of the artificial electric circuit —which is produced using microfabrication technol-ogy— to behave like a single massive electron pair. This type of a system is called a macroscopic quantum system. It is feasible to obtain physical quantities (in the forms of electric current, magnetic moment, polarization, etc.) on a much larger scale than the atomic scale while preserving quantum coherence. Other known examples of macroscopic quantum systems, besides superconduc-tivity, include superfluidity, photons in a laser state and Bose-Einstein conden-sation in dilute atomic gas.4 Quantum Node Technology68　　　Journal of the National Institute of Information and Communications Technology Vol. 64 No. 1 (2017)